1. Field of the Invention
This invention is in the field of pulping assemblies for defibering papermaking stock, particularly of high consistency. The improved pulping rotor of the present invention provides effective defibering at low power consumption as well as minimizing cavitation in the stock at high consistencies.
2. Description of the Prior Art
The traditional pulper rotor has two separate functions, first, defibering the stock suspension and second, efficient circulation of the suspension to keep it homogeneous.
There have been some disclosures in the prior art of pulping apparatus wherein stock to be pulped is introduced into a tub which is provided with a vertical axis rotor. The geometry of the rotor is such that it causes circulatory movement of the stock which causes more efficient circulation of the stock and also improves the pulping characteristics when the stock is forced between the rotor and an underlying extraction surface.
In U.S. Pat. No. 3,073,535 there is described a pulping apparatus including a rotor with vanes which project outwardly in overhanging relation with an underlying perforate bedplate. The vanes are provided with leading edge surfaces which are inclined toward the plane of the working surface between the rotor and the bedplate to force the stock into the stock working space and also to produce a rubbing action on the stock between the vanes.
In U.S Pat. No. 3,774,853 there is described a pulping device including a vaned rotor in which some of the vanes are equipped with cutter bars arranged to move past stationary bars in shearing relation. The fibers are cut by causing them to fold over the leading edge of the moving cutter and its associated vane. Each of the vanes is equipped on the surface opposite the cutter with one or more fins arranged to hold the folded over pieces of stock against centrifugal travel outwardly of the vane, thereby retaining the material in folded over position over the leading edge of the vane and the cutter until the cutter reaches the next stationary cutter where it is cut into two pieces.
U.S. Pat. No. 3,889,885 is said to be an improvement on the rotor assembly shown in Pat. No. 3,073,535. The thickness of the defibering vanes is reduced to minimize the pumping action, and separate pumping vanes are provided on the outer surfaces of some of the defibering vanes to increase the circulation effect created by the rotor during operation.
Another pulping rotor is described in U.S. Pat. No. 4,109,872. This rotor has spaced vanes each of which includes a defibering portion and a pumping portion. The defibering portion constitutes the outer end of the vane and is of relatively small axial dimension. The pumping portion has greater axial dimensions and an upper surface of convex airfoil shape. The leading face of each pumping portion is essentially planar and is inclined forwardly to overhang the trailing edge of an adjacent pumping portion so that between them they define a groove in which the stock is channeled for centrifugally outward flow. The rotor is designed to cooperate with a frustoconical extraction plate, and the defibering vane portions have their undersurfaces inclined to match the inclination of the extraction plate.
While pulping rotors of various forms have been described in the prior art, such rotors normally represent a compromise between efficient defibering and efficient circulation. Those rotors which use straight vanes approximating a radial configuration have excellent attrition characteristics but they are inefficient in pumping. Curved vanes provide better pumping efficiency but they lack the ability to efficiently defiber.